I. Field
The following description relates generally to the detection of sources of electromagnetic radiation.
II. Background
Small transmitters of radio-frequency electromagnetic (EM) energy, such as concealed electronic “bugs” and misplaced cellular phones, are often the subject for intensive searches. Accordingly, a number of technologies to detect such objects have been developed.
For example, a technology known as time difference of arrival (TDOA) of an EM signal uses a plurality of receiving locations to triangulate the bearing of the EM signal source.
A second technology, known as pseudo-Doppler, uses a multiple antenna array where the antennas are electronically switched in a simulated circular pattern to produce a simulated Doppler phase shift on the carrier frequency of the EM signal. The Doppler shift is a maximum when the velocity of the antenna is orthogonal to the wave front of the EM signal, and zero when the velocity vector of the antenna is parallel to the wave front.
A third technology, known as Watson-Watt Direction finding, is based on the directivity pattern of a multiple antenna array. The outputs of several antennas are processed vectorially to obtain an output proportional to the angle of arrival of the EM signal wave front at the antenna array.
While the above-mentioned technologies are useful, they all suffer from a number of shortcomings. For example, TDOA requires precise knowledge of the EM signal frequency, and fails when the EM signal source is physically close to the receiving system. Pseudo-Doppler and Watson-Watt techniques also require knowledge of the target signal frequency. Pseudo-Doppler similarly becomes inaccurate when its receiving antenna array is close to the EM signal source, and Watson-Watt direction finding requires highly complex and expensive processing equipment. Accordingly, new technology for the location of EM signal sources is desirable.